Nota accusativi (Latin for "signal of the accusative (case)") is a grammatical term meaning "denoting accusative case". It generally is applied to linguistic indicators of the accusative case, such as the use of the prefix "et" in Hebrew, for nouns in the accusative, which are indicated by use of the definite article (i.e. "the").
To continue with the Hebrew example (here nota accusativi is abbreviated to nacc):
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Ani ro'eh et ha- kelev. 1s see.sg.m nacc def dog I see the dog.
On the other hand, "I see a dog" is simply "Ani ro'eh kelev."
This Hebrew example is, obviously a specialized use of the nota accusativi, since Hebrew only uses the nota accusativi when the noun in question is definite.
In other languages, especially those that indicate case grammatically, there is usually a separate form (for each declension, if declensions exist) to indicate the accusative case. The nota accusativi should not be confused with such case-forms, as the term nota accusativi denotes a separate particle indicating the accusative case.